Nucleosynthesis: the births of the elements

How were the elements made? What explains the relative abundance of each element in our solar system? Below you will find a very simplified overview of nucleosynthesis, the process by which elements are formed in the burning hearts of stars. For additional information, start here.

Solar system elemental abundance

Image credit: Ringwood, 1979

The abundances of the elements in our solar system were estimated using meteorite compositions and spectral analyses (emissions) of stellar objects. These two lines of evidence, which closely corroborate each other.

Chondritic meteorites contain chondrules, rounded grains that formed as molten droplets during the solar system's youngest days. Since they formed very early and have not undergone alteration or differentiation, chondrules tell us about the formation and composition of the solar system.

The serrated pattern results because even atoms are more common than odd!

Within the nucleus, protons are arranged in layers. An even number of protons allows proton pairs to spin in opposite directions, which makes them more stable. The opposite spins allow protons to crowd closer together, resulting in higher binding energy.

This also applies to isotopes, so even isotopes are more common than odd; for example, tin-120 is more abundance than tin-119.

There are 157 stable nuclides with an even mass and even number of protons and neutrons; 53 with an odd mass and and even protons and neutrons; 50 with an odd mass, odd protons, and even neutrons; 4 with an even mass and odd protons and neutrons.

Iron has the highest binding energy per nucleon (a proton or neutron), resulting in its disproportionate abundance. Furthermore, some atoms decay to iron.

Hydrogen Fusion

Once first generations stars (the first stars to form in the universe) collapsed, second generation stars used the dead stars' helium to begin creating heavier elements via the C-N-O cycle.

Triple Alpha Process

Three alpha particles lead to carbon.

Fusion

Inside the centers of huge stars, alpha particles combine with heavier nuclides to create even heavier nuclides, up to iron-56. After 56Fe, fusion is not energetic enough to form heavier nuclides. Temperatures on the order of 109 K are needed to make silicon-28.